The solution most commonly employed for producing a reconstituted wafer consists in removing various types of chips or dies that are generally selected after having been tested and usually designated as “known good dies”. The term “chip” denotes an active electronic component, such as a bare chip, or a passive component or an MEMS (MicroElectroMechanical System). As shown in FIG. 1, these chips 10, which have connection pads 11 on a side called the active side or front side 12, are then positioned front side down on an adhesive support 20, for example by means of a pick-and-place machine. This adhesive support 20 is typically an adhesive skin 21 itself bonded to a rigid support 22. Next, the chips are encapsulated in a polymer resin of the epoxy type so as to firmly secure them. A redistribution layer or RDL is then formed, possibly in several stages, on the front side after the rigid support 22 and the adhesive skin 21 have been removed. This RDL layer, which includes tracks made for example of TiW/Cu or TiPd/Au, is formed on a dielectric layer deposited instead of the adhesive support, by dip coating or spin coating. The wafer thus reconstituted, which does not contain defective chips, can then be diced in order to obtain plastic micropackages. The wafer can also be stacked on other reconstituted wafers and electrically connected to these wafers using various known methods, the stack then being diced in order to obtain 3D (three-dimensional) electronic modules.
The encapsulation of the chips comprises:                a step of depositing the resin (by casting or by compression molding) around and possibly on the chips bonded to the adhesive support, so as to fill the inter-chip spaces; and        a step of curing the resin, thus forming a rigid handleable substrate in which the chips are fixed, the adhesive substrate then being able to be removed.        
One obvious drawback is the movement of the chips while the resin is being deposited and/or while it is curing. This is because the irreversible shrinkage and the reversible expansion of the resin after curing create relatively isotropic and predictable micromovements of the chips. These micromovements are typically between a few μm and a few tens of μm, thus possibly exceeding the required post-molding positioning tolerances that are typically of the order of 10 μm.
One solution that has been proposed consists in placing a copper rectangular-mesh lattice on the adhesive support and then in transferring the chips onto the support in the cavities located between the intersecting bars of the lattice. This lattice is thus used as a template in which the chips are placed. The resin is then deposited between the bars and then cured. This method helps to reduce the expansion, and therefore the movement of the chips, but does not enable it to be eliminated. It also has the following drawbacks:                the lattice necessarily remains in the final package, since once molded in the resin it can no longer be removed;        this limits the number of chips on the wafer because of the space taken up by the lattice; and        this requires a double dicing operation so as to make the bars of the lattice disappear.        